The choice of industrial electrical equipment is mainly aimed at long-term reliable operation and good workability in harsh industrial environments. When electrical equipment is used in explosive gas environment, special attention should be paid to prevent explosion of electrical equipment caused by sparks or hot surfaces. . Therefore, the purpose of the selection is to reduce the probability that an electrical device may cause an explosion to an allowable minimum.
The condition for the explosion is that both explosive gas and ignition conditions must exist. In a certain place, the possibility of explosive gas is high. The structure of the electrical equipment should make the possibility of generating an ignition source as small as possible. On the contrary, for those places where the possibility of explosive gas is small, the ignition source can be allowed. The probability of existence is slightly higher. Therefore, the electrical equipment and the actual dangerous conditions must be carefully matched to ensure that the safety level requirements of the electrical equipment are met on the one hand, and the unnecessary cost and system design of the electrical equipment are avoided on the other hand. The implementation of special electrical equipment for the determination of hazardous conditions and explosive atmospheres provides guidance on the selection of procedures for electrical equipment with appropriate protection levels. 2 Hazard Category 21 External Hazardous Electrical Equipment Surrounding Gas Environments are generally classified As follows: Non-explosive hazardous areas: air mixtures are not expected to appear in large quantities, and there is no need to take special precautions for the structure and use of electrical equipment. Explosion hazard sites: explosive gas mixtures are expected to appear in large quantities or may appear in large quantities, requiring The structure of the equipment or Areas where special precautions are taken Explosive hazard sites are classified as follows: Zone 0: Zones where explosive gas mixtures are continuously present or permanently present Zone 1: Zones where explosive gas mixtures may occur during normal operation Zone 2: Explosive gas mixture The area of ​​each explosive hazard that may occur, or if it occurs only for a short period of time, shall be determined by the relevant expert, and the classification map of the explosion hazard shall be used to indicate the classification of the explosive hazard site and its scope. Point out the temperature class and gas group related to explosion-proof electrical equipment. 1. In the well-designed and carefully maintained open-air equipment, the explosion hazardous area is mainly Zone 2, and its isolation zone is Zone 1, and Zone 0 is rare.
22 External Hazards of Electrical Equipment If the air is normally not present at atmospheric pressure, the internal hazards in working containers and auxiliary piping systems are not limited by the above classification of hazardous locations. However, when electrical devices are used in such equipment, these electrical devices can be compared to the hazards specified by the site classification to determine their level of danger. For example, if hydrocarbons are processed in a container under normal working conditions, and under abnormal conditions, such as factory commissioning or shutdown, the hazard is equivalent to the risk in Zone 2; when flammable substances are stored in open containers and vapors When the space is connected to the surrounding atmosphere, the danger is equivalent to the danger in Zone 0.
23 Internal Hazards of Electrical Equipment Sometimes flammable substances will inevitably penetrate into the electrical equipment casing, especially into the measuring and control instruments of the program variables. Depending on the integrity of the structure, the severity of the internal hazard that the device containing the flammable substance may form as a source of release depends primarily on whether the internal release of the device occurs under normal conditions or only under abnormal conditions.
It will be appreciated that the consequences of the release of flammable materials within the enclosure may be more severe than the consequences of the same release under open conditions, as internal release materials are typically left in the enclosure. Leaks that are not found in the open air will gradually increase the concentration in the outer shell before the atmosphere reaches flammability. This concentration can only be slowly reduced by aeration and diffusion.
On the other hand, it should be recognized that internal release typically occurs within a small enclosure, and as the release continues, the gas concentration will quickly reach the explosion limit.
Although special procedures applicable to all forms of electrical equipment are not possible, the following rules can be used as a general guide: when internal release of flammable substances is not possible even under foreseeable abnormal conditions, such electrical equipment can be considered A typical example of no abnormal release is a Bourdon tube pressure gauge with mechanical rigidity like this. Excessive metering in accordance with any foreseeable pressure does not cause the pressure gauge to malfunction. (See) Flammable substances may occur in the case of foreseeable abnormal conditions. When the internal device is released, the electrical equipment shall be considered to have a typical example of abnormal release as a thin-walled metal diaphragm or a leather tiger may cause mechanical failure due to repeated bending or accidental exceeding the rated pressure limit (see). In order to prevent abnormal release within the casing, flammable substances are The risk of ignition from the escape of the container system must be minimal. During the escape of flammable materials, operating within the operating conditions, and subject to operating conditions are subject to abnormal release of various seals, rotary seals or sliding seals, various flange joints, non-metallic flexible conduits should be assumed to be in use A continuous leak occurs after a period of time.
Where the air is thin, it is also necessary to distinguish between a restricted release and a non-limiting release. Restricted release refers to a release of any release of a flammable substance that is limited to a range of dilutions in the protective gas device. If necessary, a restrictive release can be achieved by placing a throttle valve in the conduit that transports the flammable material to the electrical equipment.
Unrestricted release means any release that is not limited by the value in the range of dilutions in the protective gas device. Usually, the internal hazard caused by abnormal release is basically the same as that of the explosion hazard in Zone 2, and the normal release is generated. The internal hazard is basically the same as the danger in the explosion hazardous area of ​​Zone 0. However, special circumstances beyond this rule may occur, such as the use of inert gas for positive pressure or thin air. 3 Actual hazard After determining the external and internal hazards of process equipment and electrical equipment, the most dangerous conditions should be selected. The basis of electrical equipment.
For example, when an electrical device whose internal hazard is substantially the same as the hazard in Zone 2 is installed in a non-explosive hazardous area or Zone 2, the internal hazard determines the actual hazard condition, and when the same electrical equipment is installed in Zone 1 or Zone 0, the external hazard Determine the actual dangerous conditions.
4 Use of industrial electrical equipment If the external hazard is classified as a hazardous area in Zone 2 or the internal hazard is similar to the hazard in Zone 2, as long as the electrical equipment does not generate arc ignition or spark ignition capability during normal operation, and no hot surface lead is generated. Burning capacity, it is often possible to use high-quality industrial electrical appliances, and mainly in non-hazardous places. The concept must consider the possibility that electrical equipment may become an ignition source under abnormal conditions, but for hazardous areas in Zone 2, such ignition sources and explosive gases are simultaneously The dangers that exist are negligible.
The applicability of industrial electrical equipment for Zone 2 shall be assessed by competent personnel. This evaluation is relatively easy because only the normal use conditions of the electrical equipment are considered. Especially in the field of small current technology, such as process instrumentation, if not all, most industrial electrical equipment can be used immediately, because these electrical equipment usually does not have thermal problems, and most of the circuits are fully enclosed if they are in normal operation. Sparks, such as accident indicating switches, whose maximum safety circuit parameters can be found in the published intrinsically safe ignition characteristics, without the safety factor not meeting the normal requirements. It is considered to have hingH. If it is impossible to use non-sparking appliances in the industry. Dedicated party bookmark1 5 explosion-proof form with electrical equipment that may be abnormally released to ensure that electrical equipment does not explode, these special methods are called "explosion-proof form"
Various types of explosion-proof forms are usually indicated by characters. The introduction of these symbols and the meaning of special explosion-proof forms are briefly explained as follows. See the explosion-proof form of the flameproof enclosure. In this explosion-proof type, the electrical equipment components capable of causing the explosion are enclosed in a casing. The outer casing can withstand the explosive pressure of the internal explosive mixture without being damaged, or prevent the internal flame from propagating to the external explosive environment through any joint surface and structural openings in the casing to prevent excessive temperature and arc and spark generation. The possibility to achieve an explosion-proof form provided by the safety h: a gas-tight gas-tight casing containing a potential ignition device for preventing mechanical damage.
Designed to: meet any design specifications and any spark or thermal effects generated electrically and mechanically or under specified fault conditions shall not cause the specified explosive mixture to ignite intrinsically safe protection in two categories: ia has specified current and The electrical equipment with the safety factor of the voltage will not be in normal operation, or the single fault that is applied, or the combination of the two faults will result in the ignition equipment not being in normal operation, or the ignition caused by the single fault applied. : The encapsulation type is in this explosion-proof shape, and the potential ignition source is potted in the flame-retardant solid insulation material. This material shall be capable of preventing breakage under internal fault conditions. In the normal operation of a non-sparking electrical appliance, neither an arc or spark that ignites nor an electrical device capable of igniting a hot surface is considered to be a non-sparking electrical appliance.
o An explosion-proof form of oil-filled electrical equipment or electrical equipment components immersed in oil. This method of impregnation causes explosions on or above the oil. p: Positive pressure type. This type of explosion protection is divided into three categories. If the pressure is greater than the pressure of the external environment to prevent the explosive gas from entering the electrical equipment, the positive air pressure is applied. If the pressure of the inert gas in the outer casing is greater than the external environmental pressure to prevent the external gas from entering the electrical equipment, the inert gas is continuously diluted by the positive pressure. (Manual ventilation) is to pass a sufficient amount of shielding gas (usually air) into the casing to disperse and dilute the flammable gases or vapours that may be generated by the release of flammable substances from the inside, thereby preventing explosions in explosive places, To prevent external ambient gases from entering the enclosure, maintain an appropriate amount of overpressure in the enclosure.
If the supply of shielding gas inside the casing cannot be maintained, in order to prevent the occurrence of dangerous situations, a visual device or an audible device alarm should be provided. Once an alarm occurs, corrective measures should be taken immediately to restore the supply of protective gas. If necessary, manually cut off the power supply of the electrical equipment. If the power supply cannot be manually cut off, it must be able to automatically cut off the power supply. The sand-filled type is an explosion-proof type filled with fine-grained material. The form is such that the arc generated in the electrical equipment casing under the intended use state cannot ignite the external environmental gas. The typical filling material is quartz sand.
r restricted breathing type An explosion-proof type ignition source is enclosed in a sealed outer casing, which is sufficient for preventing a large amount of external environmental gas from infiltrating in a limited time to generate an explosive mixture inside, and external components cannot generate an explosion s: special explosion-proof form The form cannot be completely classified into any of the above-mentioned explosion-proof forms. Several types of explosion-proof forms can be integrated into one electrical equipment. For electrical equipment with such explosion-proof form, the relevant documents should indicate which type of danger the special explosion-proof form is. Table 2 of Table 1 indicates the explosion-proof form that provides the appropriate level of safety under special hazardous conditions. In these suitable explosion-proof forms, most of them are not actually necessary, or are too good for a particular hazard. Therefore, all the details of use are required. Further analysis in order to achieve the best choice for the explosion-proof form required for special hazardous areas should further recognize that there is no mutual agreement on these issues, especially for the automatic cutting of positive-voltage electrical equipment, in case of doubt. A conservative approach should be adopted, and some requirements are expected to be relaxed in the future. Department release normal abnormal place classification 2 area no 1 area 0 area 1 2 area see no no 1 area 1 table 1 0 area 1 is 21 or 0 area 1 chart symbol Description: Ai suitable for one not suitable 2 - if internal components or The internal circuit cannot be ignited under normal conditions or has an explosion-proof form of 6,11,3, or it meets the requirements.
Note: 1 If there is no internal release in the explosion-proof component, it is suitable for both h and m explosion-proof forms. 2 In this case, this release is not important because other hazards occur first.
Table 2 Explosion-proof form "P suitable range Hazard class air positive pressure gas positive pressure air boost and dilution internal release normal abnormal place classification alarm disconnect alarm disconnect alarm cut off 2 zone 1 zone 0 zone unlimited Zone 2, Zone 1 Unrestricted Zone 2 Zone 1 Restriction Unrestricted 2 or 1 Zone 2 or 1 Zone 1 Unrestricted 2 or 1 Zone 1 Note 2 0 Zone 1 Chart Symbol Description and Notes See Table 1 6 Structural Requirements The above 11 types of explosion protection Forms include basic ways to obtain safety, but do not specify the structural requirements of electrical equipment. This is the reason for studying structural standards. The National Standards Institute, the European Committee for Electrotechnical Standardization (CENELEC) and the International Electrotechnical Commission (IEC) have published such standards. In 3, the various standard documents currently in use are introduced. The requirements for the same explosion-proof form vary greatly. The European Standard Publication (EN-50.014150020) has done a lot of work in the coordination improvement. These have been published. Standards have been accepted as national standards for all participating CENELEC countries or recognized by these countries. In addition, it has been officially required to comply with the requirements of the above-mentioned European standards and as a recognized experimental institute. At the time of the certification, the member states of the European Economic Community could not restrict the free trade of electrical equipment and made the agreement significantly improve the previous trade level. It is expected that the new design of electrical equipment will be based almost entirely on European standards. However, the use of electrical equipment that meets other standards is also possible.
Several countries have pilot research institutes, such as the UK BASEEA (British Explosion-proof Electrical Equipment Certification Authority), Germany's PTB (Federal German Institute of Physics and Technology), as an independent agency to assess whether electrical equipment complies with specific structural requirements, and It is stipulated that the electrical equipment shall withstand the test certificate institute specified in the standard to issue a “certificate of conformity†for electrical equipment that meets the requirements of the standard, and issue a “certificate of inspection†for electrical equipment that does not fully comply with all the details in the standard, but provide at least one The level of security equivalent to this standard.
The above standards are mainly suitable for electrical equipment used in hazardous areas in Zone 1 or Zone 0. For electrical equipment used in Zone 2 hazards such as n or r, the test institute is generally not required to participate. In this case, the manufacturer may provide a corresponding description. If such certificates or instructions exist, the user should collect, research and However, as long as they do not involve other aspects of security dependencies, such as installation and maintenance procedures, their value should not be overvalued. Therefore, it is unusual for users who know the explosion hazard in their factories to use reliable and competent manufacturing. The electrical equipment produced by the factory does not use a formal certificate. This situation may be encountered in the special case of a new product (eg, when the qualification test has not been completed), or where a small amount of special electrical equipment is required, and it is not actually necessary to perform a time-consuming and costly qualification step.
Table 3 Structural requirements for use of the International Electrotechnical Commission, European Committee for Electrotechnical Standardization, British Explosion, Netherlands, France, United States, Forms, New Standards, Old Standards, General Requirements, Part 1, Part 5, Part 2, Part 6, Part 4, Part 7, Part 3, Part 2 Part 3, Part 4 Note: 1 These national standards are equivalent to European standards 1980) only include two types of explosion protection, excluding European standards. The D1N national standards and VDE regulations are being studied in German publications.
In the process of improving preparations.
Selection steps According to the general considerations in the hazardous conditions and the above-mentioned explosion-proof forms, the relevant selection steps are summarized as follows: Step 1: Study the site classification map and install the electrical equipment in a non-explosive hazardous area, such as a switch room or a control room.
Step 2: Most of the remaining electrical equipment should be installed in hazardous areas in Zone 2 as much as possible. In this case, as long as it cannot be ignited under normal working conditions, first consider the use of general industrial electrical equipment. Step 3: In the second step, Where applicable, the appropriate possibilities for explosion protection should be selected from Tables 1 and 2. The specific hazards given in Tables 1 and 2 should be preferred for specific hazards, but not for more hazardous locations.
Step 4: When selecting “Flameproof Enclosure†and “Intrinsically Safeâ€, determine the appropriate temperature class for the electrical equipment and the appropriate gas group. Step 5: Select the type of explosion protection, the appropriate temperature rating and Appropriate gas groups (if necessary) to determine the industrial effectiveness of electrical equipment. If it fails, choose an electrical equipment that is more suitable for more hazardous.
Step 6: When there are several possibilities, the equipment cost and operating cost should be compared to make the final choice.
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